1. Field of the Disclosure
The present disclosure relates to a semiconductor memory device, and more particularly, to a semiconductor memory device capable of storing multi-bit information by including a variable resistance device with a solid electrolyte in a 3D structure in a storage node.
2. Description of the Related Art
Generally, variable resistance devices are devices in which a resistance changes according to conditions. In a conventional phase-change random access memory (PRAM), which is one type of a nonvolatile memory, a variable resistance device with a phase change material such as GST (Ge—Sb—Te), which has a resistance that varies according to a phase change caused by a change in temperature, is used.
In the PRAM, the phase change material of the variable resistance device crystallizes due to Joule heating or becomes amorphous, thereby changing the resistance of the variable resistance device to store the information. However, generally, a high temperature of 900° C. or greater is required to change the phase of the phase change material. Thus, much heat is generated in storing or erasing information.
U.S. Pat. No. 6,487,106 discloses a memory device with a programmable microelectronic device. FIG. 1 is a cross-sectional view of the programmable microelectronic structure. The variable resistance structure includes a substrate 110, and a first electrode 130 formed on the top of the substrate 110. An insulating material layer 150 with a through-hole formed therein is disposed on the top of the substrate 110. A solid electrolyte 140 and a second electrode 120 are sequentially formed in the through-hole.
When a voltage higher than a predetermined threshold voltage is applied across the first and second electrodes 120 and 130, metal ions of the solid electrolyte migrate and form an electrodeposit. As the electrodeposit forms, various electrical properties change such as the resistance between the first and second electrodes 120 and 130. Information is stored in the variable resistance structure by considering such changes as the resistance between the first and second electrodes 120 and 130.
However, the variable resistance structure illustrated in FIG. 1 cannot be miniaturized to a size presently required. FIGS. 2A and 2B are views of a method of forming the programmable microelectronic structure illustrated in FIG. 1. The problem with the variable resistance structure will be described in detail by explaining the method of forming the variable resistance structure.
First, as illustrated in FIG. 2A, the first electrode 130, a solid electrolyte layer 140′, and a second electrode layer 120′ are sequentially formed on the substrate 110. Then, as illustrated in FIG. 2B, needless portions of the solid electrolyte layer 140′ and the second electrode layer 120′ are removed through a photo masking process to form a solid electrolyte layer 140″ in a block shape and the second electrode 120. The insulating material layer 150 is formed on the portion where the solid electrolyte layer 140′ and the second electrode layer 120′ are removed and surrounds the solid electrolyte layer 140″ and the second electrode 120.
In this case, in the process of etching and removing the needless portions, the second electrode 120 is undercut. That is, the circumference of the solid electrolyte block is over-etched up to a distance “d” shown in FIG. 2B. To increase the integration of a semiconductor device, the width D of the variable resistance device must be as small as tens of nanometers. Since a typical solid electrolyte and an etchant have over-etched widths of about 10 nm or greater, it is difficult to manufacture the variable resistance structure in the size desired by the market. In addition, the possibility of damage to the sides of the stacked structure during an etching process increases. In more detail, the sides of the components become nonuniform, and thus it is difficult to obtain desirable electrical properties. Therefore, in order to obtain a highly dense memory device, a variable resistance device with a new structure that can be miniaturized to a nanoscale is required.